27 July 2014

#9. Carbohydrates - Polysaccharides

Molecules contain hundreds/thousands of monosaccharides linked into long chains.
Molecules are enormous --> the majority do not dissolve in water --> good for storing energy (starch and glycogen) or for forming strong structures (cellulose).

1. Storage Polysaccharides

Glycogen (in animals and fungi)
  • Made of α-glucose molecules linked together by glycosidic bonds
  • Most of the bonds are α1-4 links (C1 on one glucose + C4 on the next) 
  • There are some 1-6 links, which form branches in the chain. 
  • The bonds can be hydrolysed by carbohydrase enzymes to form monosaccharides, used in respiration. 
  • The branches increase the rate of hydrolysis.
1-4 and 1-6 links in Glycogen.

Starch (in plants)
  • A mixture of amylose and amylopectin. Both forms of starch are polymers of α- Glucose. Natural starches contain 10-20% amylose and 80-90% amylopectin.

  • Amylose molecule is a very long chain with 1-4 links. The chain coils up into a spiral, held in shape by H bonds between the glucose units. 

    • Amylopectin differs from amylose in being highly branched. Short side chains of about 30 glucose units are attached with 1- 6 linkages approximately every 20-30 glucose units along the chain. 

    2. Structural polysaccharides

    Plant cell walls contain the polysaccharide cellulose:
    • Made of many β glucose molecules, linked by β 1-4 links. 
    • Adjacent glucose molecules in the chain are upside-down to one another.
    • The chain is straight (not spiralling). 
    • H bonds between chains --> very strong microfibrils --> cell wall will not break easily if the plant cell absorbs water; difficult to digest (few organisms have enzyme that can break the β 1-4 bonds).

    Syllabus 2015 

    (c) describe the formation and breakage of a glycosidic bond with reference both to polysaccharides and to disaccharides including sucrose;

    (d) describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate these structures to their functions in living organisms; 

    Syllabus 2016  - 2018

    c)   describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose

    d)   describe the breakage of glycosidic bonds in polysaccharides and disaccharides by hydrolysis,  with reference to the non-reducing sugar  test

    e)   describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate  these structures to their functions in living organisms

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